The American Society for Testing and Materials publishes standard tests for many materials. One such test, Standard Method for Penetration Test and Split Barrel Sampling of Soils ASTM No. D-1586-84,describes the standard procedure for driving a split barrel sampler into the ground to obtain a representative soil sample and a measure of the resistance of the soil to penetration. This test is known in the industry as the "Standard Penetration Test" and is used extensively in a great variety of geo-technical exploration projects and ecology surveys. ASTM No. D-1586-84 p. 2. In this test, a 140 pound (63.5 kg) hammer is dropped on a split barrel sampler from a height of 30 inches (76 cm) above the barrel to force the barrel into the ground. The barrel, having been forced into the ground, picks out a plug of soil which is removed and analyzed in a laboratory.
While driving the barrel into the ground, the operator keeps track of the number of hammer blows needed to drive the barrel one foot (30 cm) into the ground. The number of blows per foot (30 cm) of penetration is called the N count or blowcount. Many widely published correlations relate this blowcount to the engineering behavior of soil and foundations. ASTM No. D-1586-84 p. 2.An accurate blowcount is essential to valid test results. The hammer must be dropped from 30 inches (76 cm) above the barrel before every blow to get an accurate blowcount. Although ASTM No. D-1586-84 allows the use of an automatic drop hammer system, a simple cathead is usually used. An operator simply winds the hammer line up on the cathead until it has lifted the hammer about 30 inches (76 cm). Carelessness and impatience introduce large error into the lift of the hammer. Therefore, blowcounts are likely to be very inaccurate. ASTM No. D-1586-84 does not describe any automatic drop hammer systems.